The invention relates to a device for use in an offset printing press for supporting and positioning of printing cylinders of the printing press exchangeably therein.
For the resetting of an offset printing press for variable sizes to another format it is known to exchange a printing assembly or even a format assembly in its entirety per printing tower. Such a format assembly is composed of a housing in which the printing cylinders belonging to a particular format are accommodated in a fixed manner, and in the case of which provision is also made for a gear wheel transmission between the successive printing cylinders, means for enabling the displacement of one or more cylinders axially, and further adjusting means. These format assemblies have to be placed in a printing tower by means of a lift truck, after which the format assembly is fitted in the correct place in the printing tower and is locked therein, which operation can be carried out manually or by means of a pneumatic system fitted in the printing tower.
If a number of different formats have to be available for a printing press with only a minimum number of successive printing towers, it will be clear that already at that stage the number of format assemblies required very soon mounts up. The difference between the format assemblies is substantially the diameter of the format-dependent printing cylinders and that of the corresponding drive gears; the housing and the further adjusting means fitted therein are always virtually the same.
U.S. Pat. No. 3,611,924 shows an offset printing press wherein the image printing cylinder and transfer printing cylinder are changeably supported per se to permit the use of cylinders of different diameters for printing sheets of different length. The transfer printing cylinder is mounted on a pivoted bearing arm that can be rotated toward and away from the image printing cylinder and from an impression printing cylinder. After the transfer printing cylinder has been rotated away, the image printing cylinder can be removed via a horizontal slot. An adjustable eccentric bearing is provided for the transfer printing cylinder to be able to adjust it with respect to the other cylinders.
A disadvantage with this known device is that it is relatively complex and expensive, and that its printing quality leaves to be desired. The adjusting of the eccentric bearing is time-consuming and must be performed each time after a new format of cylinders is placed. It frequently occurs that the transfer printing cylinder lies with different amounts of pressure against both other cylinders, leading to local enlargements or reductions of image dots transferred to the transfer printing cylinder and/or printed on the sheets. Printing on sheets with different thickness, has the same disadvantage of having to adjust the eccentric bearing, and also frequently leads to distortions of printed images. Furthermore, the axes of the several cylinders can not be brought in line with respect to each other for both large and small formats of cylinders. Because of this the supporting means for the cylinders must be relatively strong, and also a deterioration of printed images occurs, for example if small irregularities are present on one of the cylinders. In order to be able to remove the image printing cylinder, a large rotation of the pivoted arm with transfer printing cylinder has to be made. Should only an image carrier plate on the image printing cylinder have to be changed, then this plate is barely accessible.
The object of the invention is to overcome the above mentioned disadvantages, and more in particular to provide a simple and user friendly device with which only the format-dependent cylinders per se have to be changed while the necessary means for permitting correct positioning of the cylinders relative to each other are accommodated in the printing tower, and with which a good image printing quality can be achieved.
This object is achieved with a device according to claim 1. The device comprises first and second bearing arms with supporting elements for permitting the accommodation in an exchangeable manner of at least two of the printing cylinders, and first and second actuating means for rotating of the first and second bearing arms respectively for positioning the at least two printing cylinders. This ensures that when there is a changeover to a different format only the format-dependent cylinders per se need to be changed. The cylinders are subsequently placed in the correct position relative to each other by the actuating means and bearing arms. Thus, according to the invention, provision is made for at least two of the cylinders to be fitted on or near ends of bearing arms that are rotatable about an axis, the actuating means being able to rotate the bearing arms and supporting elements through a predetermined angle.
The invention has the advantage that the mutual pressures between the cylinders can accurately and easily be adjusted by minor rotations of the bearing arms with respect to each other, making it possible to avoid distortions of images printed. Also different thickness in substrate to be printed is no longer a problem. By means of a suitable mutual adjustment of the positions of the bearing arms it is possible to each time have the cylinders approximately lying in line for different sets of formats. This also leads to an improved image quality, and makes it possible to construct the bearing arms, positioning elements, etc. somewhat less strong. Each of the cylinders is readily accessible, even when only an image plate or a compressible transfer layer has to be changed.
Each of the cylinders supported by a bearing arm is preferably supported by bearing arms placed on both sides, the actuating means preferably also acting upon both bearing arms. This latter situation is preferred, since the cylinders can then not only be positioned always exactly parallel relative to each other, but can also be held securely in that position during operation. Also the actuating means for each bearing arm, preferably can be actuated separately. Thus, for example, one of the bearing arms can be loosened from the cylinder, then be rotated up or downwardly, after which the cylinder can be removed from the other bearing arm by taking it out sideways in the direction of the cylinder axis.
A further development provides for the first actuating means to be rotatably connected to a fixed point and the first bearing arm, and for one or more further actuating means to be rotatably connected to the first bearing arm and a further bearing arm. A possible embodiment of this is to have the first actuating means acting upon a bearing arm of the transfer printing cylinder and two further actuating means acting upon a bearing arm of the transfer printing cylinder and upon a bearing arm of the image printing cylinder and the impression printing cylinder, respectively. The advantage obtained by this is that the positioning of the cylinders is less dependent upon one margin of error.
A possible further variant of this is to place one of the cylinders, for example the image printing cylinder, in a fixed position and to provide the other cylinders on bearing arms co-operating with each other, that is to say the first actuating means actuable relative to a fixed point, while the second actuating means are actuable relative to the rotatable first bearing arm. Thus the construction is very simple. Means for axially moving the image printing cylinder can advantageously be provided onto fixed supporting elements. Furthermore in most offset printing presses inking mechanisms are already adjustably mounted, making it possible for several formats of image printing cylinders to be placed against them.
According to a further development, the means for permitting a bearing arm to rotate through a predetermined angle comprise at least one adjustable distance device. Such an adjustable distance device can act near the end of the bearing arm, where a supporting element for a cylinder may also be situated. However, it is also possible to act upon the bearing arm on the other side of the centre of rotation; this depending on whether it might possibly work out better, on account of the available space in a printing tower.
The adjustable distance device can be, for example, a screw spindle, a hydraulic cylinder or a gear rack system. A screw spindle has the advantage that it is rigid, cheap, and that its position is accurately computable, making it possible to adjust the relative positions of the cylinders based on position of the spindle, instead of based on pressure differences coming from contact between the cylinders. A hydraulic cylinder has the advantage that it is easily protectable against overpressure leading to damages or injuries. In the case of use of a screw spindle with an element to be moved along it, it is possible, for example, to opt for a screw spindle with trapezoidal thread, or a ball circulating spindle, a ball circulating spindle being preferred on account of the low static and dynamic friction. The drive of a screw spindle can be achieved by, for example, an electric motor, a reduction gearbox fitted between the motor and the driven part of the spindle ensuring that the desired torque is supplied and the driven part can always be moved over a minimum distance.
In order to be able to take the printing cylinders into the correct position relative to each other and possibly also relative to the inking rollers, which is preferably carried out by calculating the position and the desired change, a number of values must be known, such as the format of the cylinder and the last position taken up by the supporting elements. It is important here always to know exactly the movement of the driven elements along the spindle.
For this purpose, provision is made according to the invention for a detection device to be connected to the drive for the driven element or to the driven element on the screw spindle, for detection of the angle of rotation of the drive shaft or the driven element on the screw spindle. Measuring or determining the angle of rotation ensures that the movement of the driven element along the spindle is known, and consequently so is the rotation of the bearing arm, and therefore also the new position of a supporting element.
It is possible, instead of the abovementioned detection devices, to provide optical or electromagnetic means to permit detection of the position of a supporting element relative to a scale.
In the case of the device according to the invention, in order to avoid complicated transmission constructions or having to change gear wheels for the changing positions of the printing cylinder for differing formats, provision is made for at least two of the printing cylinders to have their own drive for the purpose of rotation about their own axes. The drives are directly linked to the control system of the printing press, which makes it possible to synchronize the printing cylinders in a printing tower and also successive printing towers with each other in a simple manner.
In principle, it is possible to provide for the drive of the impression cylinder by connecting the latter by means of, for example, a toothed belt transmission to the drive for the rollers of the inking train in the printing press or module thereof. The speed of the drive obtained in this way is sufficiently constant for the impression cylinder.
The supporting elements for the printing cylinders can be composed of cones which are situated on either side of a printing cylinder and by axial displacement can be taken into and out of engagement with a printing cylinder.
However, a through-running mandrel can be used as the supporting element, over which mandrel the cylinder can be pushed and on which it can be clamped by means of a mechanical, electrical or pneumatic clamping device. This has the advantage that the printing cylinders can be taken into and out of the printing tower in the axial direction, and thus at the side of the printing tower, and that manoeuvring between successive printing towers is not necessary. An additional advantage is that the weight of the cylinders can be lower than that of the printing cylinders that have to be wedged between two cones. If light materials are then also used for the manufacture of the printing cylinders, it is even possible to change over the cylinders by hand without aids.